The theory that emo music is really cheerful and fun, until you listen to the lyrics then it becomes sad. Usually occurs because of emo bands inablity to come up with good lyrics.

Foster: "Dude this song face down is awesome! it makes me so happy"
Laura: "You realize that song is about a guy who beats his girlfriend?"
Foster: "Aww...now it makes me sad to listen to that song"
Laura: "Yea Its the Theory of Emo Music"